The inventor of the present invention has proposed an earth material (U.S. Pat. No. 5,500,245) subjected to water-repellent treatment for prevention of weed growing at specific regions such as median strip of a highway. The earth material is made by a method including the steps of: heating a starting earth material for drying, wherein the starting material is selected from the group consisting of soil, sand, gravel, pebbles, silicon-containing combustion ash of household refuses and silicon-containing combustion ash of industrial refuses; preparing an emulsion of silicone oil in water; soaking the earth material with the emulsion; and heating the soaked earth material for drying.
In general, the water-repellent earth material has a grain size of no greater than 10 mm. In use, the earth material is spread over the surface of a selected site to form a surface layer. The surface layer will be made to prevent or allow penetration of water depending on whether the water-repellent earth material contains fine dust particles or not.
Specifically, when the water-repellent earth material includes fine dust particles having a grain size smaller than 2 mm the distances between the earth material grains are made small. In this instance, the surface layer made of such an earth material will become resistant to water penetration. Therefore, rainwater is not allowed to penetrate downward through the surface layer, and water remaining on the surface layer will evaporate immediately due to solar heat and wind. Thus, no water is supplied to the inside of the surface layer. As a result, weed seeds carried onto the surface layer fail to germinate, so that weed growing is prevented.
On the other hand, when the water-repellent earth material does not include fine dust particles having a grain size smaller than 2 mm (this may be a case where fine dust particles smaller than 2 mm were removed beforehand by sieving for example), the resulting surface layer exhibits water permeability. Thus, water is absorbed through clearances between the grains of the water-repellent earth material and arrives at the bottom of the surface layer. Further, if there is some water remaining in the surface layer halfway therethrough, it will evaporate immediately. As a result, no water is supplied to the weed seeds carried onto the surface layer, thereby preventing the weed growing.
As described above, the surface layer made of a water-repellent earth material, whether allowing water penetration or not, can check the germination of weed seeds carried onto the layer. However, there is also a problem that weed growing at the site is not completely prevented. This is because weed seeds existing beforehand in the soil below the surface layer of the water-repellent earth material may germinate and grow upward through the surface layer. In particular, when the surface layer of the water-repellent earth material allows water penetration, the above problem is not avoidable since water supply to the weed seeds is possible.